A dual-energy gamma ray transmission method is currently widely used for measuring an ash content of a coal. By measuring attenuation coefficients of gamma rays with two different levels of energy from a dual-energy gamma ray transmission measuring device with regard to a coal flow, the ash content of the coal is computed, and thus a calorific value of the coal may be derived from the ash content of the coal. This method is real-time, rapid, convenient to operate, and suitable for a coal utilization site which has a fixed coal source.
However, a disadvantage of the method lies in that a measuring result is greatly influenced by a change in a content of a high-atomic-number element in the coal. For example, the measuring result of the ash content is directly influenced by a change in a content of an element such as Fe or Ca in the coal. For a coal utilization site such as a thermal power plant, which has complicated coal sources, because the coal used comes from a plurality of mines or production regions, in general, the change in the content of the high-atomic-number element in the coal is larger, and thus an error will be larger when this method is used to measure the ash content of the coal. Moreover, it is difficult to solve the problem by calibration, because a calibrated parameter used for computing the ash content is accurate only for a coaly being calibrated, and once the coaly has varied, a larger error may be generated when the calibrated parameter is used to compute the ash content. However, in practice, a change in the coaly is usually irregular and difficult to predict, and therefore it is difficult to use this method for measuring the ash content in a case of complicated coal sources.